Frequency or phase modulation receiver



NOV. 29, 1938. M. G,- CROSBY 2,138,341

FREQUENCY OR PHASE MODULATION RECEIVER Filed May 26, 1957 4 SheetS-Sheet 1 f FREQUENCY ff fc f2 FREQUENCY INVENTOR M6.C`R0$BY )KZ/1mm,

l ATTORNEY Nov. 29, 1938. M. G. CROSBY 2,138,341

FREQUENCY OR PHASE MODULATION RECEIVER Filed May 26, 1957 4 Sheets-Sheet 2 I l I I 34 *fl fc z L INVENTOR M. G. @R055 Y BY ffm ATTORNEY Nov. 29, 1938. M. G. CROSBY EREQUENCY OR PHASE MODULATION RECEIVER Filed May 26, 1937 4 SHfets-Sheet 3 Ann-M YVVVVV AAAAAAAA INVENTpR M. 6. CROSBY BY MZ ATTORNEY Nov. 29, 1938. M. G. cRosBY FREQUENCY OR PHASE MODULATION RECEIVER Filed May 26, 1937 4 Smets-Sheet 4 INVENTOR M G. CRBV BY wW/W ATTORNEY Patented Nov. 29, 1938 UNITED STATES-- PATENT OFFICEn FREQUENCY OR PHASE MODULATIONv RECEIVERPv Murray G. Crosby, Riverhead, N. Y., assignorv to Radio Corporation of America, a corporation of Delaware Application May 26, 1937, Serial No. 144,778 11 claims. `(c1. 25o-2o) of the carrier frequency, but is constant for the frequency' deviations on the other side of the carrier frequency. By utilizing two such filters of opposite slopes, followed by linear detectors andiproper combining circuits, each detector conl-.tributes one half of thezmodulation envelope to theioutput ina mannerisimilar to class B pushpull amplification.

In the prior artv offrequency modulation recep-V tion the modulationis usually detected by first converting the frequency deviations of the carrier to amplitude `deviations ofthe carrier and then detecting these-amplitude deviations in the normal manner of amplitude modulation detection. This conversion is effected bypassing the frequency modulated-'wave through a circuit'whose' output varieslinearly with the frequency of the input such as the side of avresonance -curve or a suitable lter network. Receivers of this type are described in' Usselman Patent '#1,794,932 issued .March 3, 1931; Crosby Patent'#2,060,6ll issued November l0, 1936; Crosby United States application #25,026 led June-5, 1935; Crosby United States application #25,231 led June 6, 1935 now Patent #2,087,429-dated July 2D, 1937; and Crosby United States application #114,894, filed Dec. 9, 1936.U It is also possible to utilize a time delay network-or radio-circuit in such a manner as to superimpose a phase deviation on the wave which is proportional to the frequency deviation caused by the signal so that the phase deviation may be detected by means/0f a phase detector. Receivers of this type are described in Crosby UnitedStates applicationl #618,154 led June 20, 1932; Crosby'United Statesapplication #616,803 led June 13, 1932 "now Patent #2,065,565 dated December 29, 1936; Crosby United- States application li1525,026.flled`June 5, 1935; and Crosby United States application #25,231 filed June 6, 1935 now Patent #2,087,429 dated July 20, 1937. In describing my invention in detail and its relation to the prior art, reference will be made to the attacheddrawings wherein,

Figure 1 shows a diagram of a receiver arranged in accordance with the present invention. In this circuit the converting units or lter circuits., are shownlas rectangles and their respective characteristics are shown in the rectangles;

Figure Zillustrates a modification of the arrangement of Figure V1. In Figure 2,'for the sake.

of brevity, portions of the complete receiver as illustrated in Figure l, have been omitted;

Figures 2a andZb show modifications of the arrangements of Figures l and2. In these modications also, for thefsake ofbrevit'y, the-initialY stages of the receiverhave been omitted; f

Figure 3 illustrates the details of additional. converting circuits including lters having mod'- ied characteristics and their connections to rectifiers to reproduce the signal. The ltersshown in detail in Figure 3 may be used in the circuitsv of VFigure 1;

Figures 4 and 5 show graphically the characteristic of the filters of Figure 3;

Figures 6 and '7 illustrate filter circuitsawhich may be used in the rectangles 9 and 10 of Figure 2';

Figure 8 illustratesv the characteristic of a converting-or filter circuit asv-known in the prior art and illustrated in some of` the applications referred Vto hereinbefore; while,

Figure 9 illustrates `graphically in -generalthe lter characteristic of the present invention.'

In allof the receivers of the-prior art the converting` circuits operate to produce in theirv output, waveenergy the amplitude of whichis substantially linearly proportional to input frequency'or phase deviation ofthe carrier Wave-over the normal. radio frequency or intermediate frequency channel of the receiver. That is,A the amplitude might have a value of l at the carrier frequency, a Value ofy 2 at one'edge of the channel and a value equal to zero at the other edge of thechannel; In the receiver of this disclosure the prior- Graphically, the receivers of the prior art having characteristics as shown in Figure 8, while the receiver of this disclosure, in `one of its modifications, has aV characteristic as shown in Figure 9 and in all of its modifications follows the teaching of the graph of Figure 9. AIn both graphs f1 to f2 is the channel and fc is thecarrier frequency.

In Figure 8 if the carrier frequency swings ineither direction from the mean carrier frequency, there results substantially linear changes in amplitude of the output energy. In accordance with this invention as the carrier frequency decreases (see Figure 9)` the output is substantially constant whereas when the carriery frequency increases the output varies substantially linearly with change in frequency. While Figure 9 illustrates the general principles of the present invention by illustrating one modification thereof various other filter characteristics which are modifications of the arrangement of Figure 9 may be used and some of these have been described hereinafter.

In order to receive both halves, e. g., the carrier and both sidebands of the phase or frequency modulated wave, two filters are employed as is'- shown in Figure 1. The receiver of Figure 1 may be of the radio frequency type or may be as illustrated, of the heterodyne type. In the latter case, the receiver comprises any energy intercepting device such as an aerial I, a radio frequency amplifier 2, a heterodyne detector comprising an oscillator and a rst detector 3,-an intermediate frequency amplifier and band pass filter 4, and in addition, an amplitude limiting device 5. The elements I to 4 inclusive are so Well known in the prior art, such as for example the cases referred to hereinbefore, that they need no illustration here. The limiter 5 may be of the overloaded tube type or of the automatic Volume control type. The limiter 5 per se has been described in the applications referred to hereinbefore and needs no detailed description at this point. kThe output of the amplitude limiter 5 Apasses through the primary Winding of a trans- 1 Coupling tubes 'I and 8 have their anodes and cathodes connected as shown, to the inputs of two filter or converting circuits included in units 9 and I0. Filters 9 and I9 in the coupling tube plate circuits have opposite slopes with substantially zero output at the carrier frequency as is shown in the graphs in the rectangles 9 and I0 representing filters with characteristics as shown by the said graphs. The outputs of the two filters are fed to the anodes and cathodes of linear diode detectors II and I 2. Rectification of the modulated waves in II and I2 produce currents of varying intensity therein which, in turn, produce potential drops across resistors 34 and 36 which vary substantially in accordance with the modulations in amplitude of the wave in the outputs of 9 and I9. These potential varia.- tions are impressed by coupling c-ondensers and resistors 38 and 39 on the control grids of amplier tubes I3 and I4 and from the anodes of said tubes on the primary windings of a'transformer I6, the secondary of which may be connected with any utilization circuit connected with jack I7. The switch I5 permits the anodes of tubes I3 and I4 to be connected in push-pull or in push-push relation. i

As the frequency of the llimited intermediate frequency wave in the output of 5 deviates with modulation to a frequency higher than the intermediate frequency carrier frequency the output of filter 9 changes'substantially linearly with changes in frequency whereas the output of filter I0 remains substantially constant. Consequently diode detector I I will reproduce that parq nection of switch I5 with the primary winding of transformer I 6, the two halves of the modulated wave are combined in proper phase so that the full wave of the modulation is received in a manner similar to Class B amplification.

The filters in 9 and I0 may take any form. Preferably these lters are as illustrated at 45 and 45 in Figure 3 and produce characteristics as shown in Figures 4 and 5. These filters each comprise series parallel tuned reactances LC and LC1 and LCz and LCi and shunting reactances C1L and LC4 and CiLi and L05. The terminals of each network is shunted by resistances 43 and 48 and 41% and 4l as shown. These filters are standard band-elimination lters. They would normally be used to eliminate the range of frequencies between Fc and F2 of Figure 4, or between F2 and Fc of Figure 5. The carrier is tuned to one edge of theband elimination range so that the frequency modulation swings the frequency into the band elimination range on one side of the carrier and out of the band elimination range on the other side of the carrier frequericy. The filters are of a standard design taken from the book Transmission Networks and Wave Filters, by T. E. Shea.

Figure 2 shows an arrangement wherein output at the carrier frequency (of intermediate frequency here) is substantially maximum and frequency or phase deviations produce a decrease in the lter outputs instead of an increase. In rectangle 9 the lter has a characteristic such that as the frequency increases from the carrier frequency the amplitude at theoutput decreases substantially in proportion to increases in frequency, whereas as the frequency decreases from the carrier frequency the output of filter 9 remains substantially constant at a value substantially equal to maximum output. In filter I the reverse operation takes place on change of frequency from carrier frequency. The circuit of Figure 2 is otherwise similar to that shown in Figure l. In Figure 2 the circuit elements, prior to the outputs of coupling tubes 'I and 8 have been omitted, while the circuits following the outputs of the filters are the same as in Figure l and need no further descrip-tion here.

Figure 6 shows a standard low-pass filter which may be used in the rectangle of 9' of Figure 2. The standard high-pass filter of Figure 7 may be used in rectangle I0. These filters may be designed so that their cut-01T frequencies occurred at the carrier frequency and their frequency of maximum attenuation occurred at the frequency f2 in the case of the llow-pass filter of 9 or f1 in the case' of the high-pass filter of rectangle I0'. Thus, as the frequency is modulated, it is either modulated into the pass band of the lter or towards the frequency of maximum attenuation and minimum output. T'his receiver operates quite similar to the one of Figure l except that the half wave of audio voltage produced by each detector is due to a decrease in the energy applied to the detector instead of an increase as in the receiver of Figure 1.

The receiver of Figure 2 also enables the reception of amplitude modulation by using the pushpush connection of switch I5 and switching out the limiter 5 preceding the coupling tubes. In the receiver of Figure 1 the carrier is substantially of zero amplitude so that the amplitude modulation cannot be received. This feature becomes an advantage in the receiver I in-that there is a tendency to reject amplitude modulation noises. In the receiver of Figure 2 the carrier is substantially of maximum amplitude Whereas .the side frequencies are .either Vof corr-24` stant amplitude for* decreaseiwitlr-frequencyzf Various combinations.offV thewltersof Figuresil. and `2 maybe ,-utilizedto produce` advantages with; 5 .v regard, tonoise. andv V'interference elimination; asf: Well as` characteristic; improvement., Eer: instance; filters .95. and l maybe-utilized'.withQthe j push-push connection.v of switch... I5; This.; are rangement or modification.mayl besas: illustrated; h-in Figura 2m.. Also. yfilters I 0.'and;9;may;be Vused. with the push-push connection forthel'anodesfofz" tubes-.l z-andal 4. 'Ihismodicationhas been illus.- tratedimFgureZb.-

Whernrcceivers LofzFigures ZaandxZhuse arcom-k ,bination zof allowepassgfrlter anday band-elimina. tionrlter or :thecombinatioiaoff. a highi-pass. lter and: 'la band.elirninatiorr4 filter. theyV have v as an". advantage the factv :thatzinterference: on; one. side ofthe acarrerrfrequencyfis; reduced.` In: the -case'A glgof-i Figure 2a; thezlter ofq has an-output :which decreases: towards zeroas thefirequency fziszapproached.- Consequently, thef-closer-fthe inten-- ference is :tothe 4virequencygya; the :more :Will it :beeliminated bye this, filter; The. other filter` of rectangle ,i 0v has --zero or fver-y; low: output inthe total rangefbetweenltheecarrierfrequency and f2. Thus, bothlters cooperate toreduce interferenceY on-.onefside offthezcarrier-frequency; If Yit happense-thatthefinterference is on..the otherside of the carrier frequency, the highefrequency; oscillator,A of thea superheterodyne; receiver mayj be tuned to-fthe other l side :of l:thesignal Thisv will heterodyne--the- Ainterference ,to 1 the; other Y side of` the intermediate frequency; carrier frequency. 5 VKAnother alternativezwould .bef-tofusefthereceiver of. Figureb which.discriminatespagainst7 the iny terferencefor .the-.range ,between ,f1 andxthefcar riertfrequency..

The detailed. operation of. the.. receiver ofv Figi- 4zoure, 2a.,is as ffollows: As *,thevcarrier is. modulated. towardstherequency f1, the output of-.thez-lterr of tl-1`erectangle Si. will.,remain unchanged. and Y there ,willbernoaniplitude change'tobe detected by thedetector, H; Onthe other handtheout a higher. level in linear accordance Withthe fre.- quency'deviation. This'amplitude change willbe. detected by detector l2 so that, an output is producedwhichjs in linear accordance with the sigg nalling potentials. Thus, detector I2. furnishes. the half Wave of thesignalling Wave which deviates the frequency frorn'f` to f1. When Vthe frequency isdeviated towards f2, the output-of the^ filter-of '9'Y changes anddetector'll furnishesthat 55,' half Wave of energy. Hencegfilst one detector and v'-then; the other operates to furnish theopposite halves of the signallingviave-A in the 'same manner that class B' audio amplification oper-- ates;V Since the detectorfor lter 9 produces an woutput-"due-to' a reductionfoffthe amplitude of` theA energyA fed to the detector,` and 'the `detector ofA-'iilter- Illproduces itsfhalfrwavedue to an'increase-of fthe l energyl fed to the detector,l the *two detector--outputs -are alreadyA oi` therequiredv op'- =1P0S13e phase. to-'make-'therrr add up-as afullwave.

Consequently the push-push i or. parallel connection-of the switcli- -l5may be used.-

The-operation of the-receiver of 'Figure 2b'is the .sarneas th'eoperation ofthe receiver. of Fig- `lure 2ur except that the filters are arranged so that the- -interferenceelimination.v4 takes place for fre-.- quenciesf-on the other 1side of "the carrierl frequency. In all other respects the operation may. bethe-same.'V

5 putbfgthefllter ofrectangle lschangestowards..

The circuiti of:'*'Figure.3 .'fshows'; armere specific 1.

embodiment of; thel demodulator of t fthis discloe Sura. Im this; modification the; units .l :to: zinclue. sive outhe-systemhave been omitted.` The-input:

electrodes `of couplingftubes 40 and 42 'areshovvrr asfbeing cap-acitively'coupled' to the output of the* limiter-5;l It-will be understood thatthese tubes-I EBland-42fcorrespondto'couplingtubes 1 and 8 ot! Figuren', and: the circuits are interchangeable;-

Herc; as inVv the prior: modincations, the controla gridsi'oftubes 4zand 42" are connected in parallel tol-beenergizedv in phase v:by the. limitedv phase` or# l frequencyl m-odulatcdfwavey energy. 'Ihev anodes l of? tubesA 40 and:A 4-2 are.; connected' together," asf Shown by` way vof impedances` I3` and 44` and are*v coupledto the control gridso'f tubes 49 and 50-by way."v of.' lterf circuits 45- and 46e. respectively. Dampingresistors 41and=48;are= connected in" shunt-to.;theqoutput ofthelter circuits:45 andz.. 4S=respectively; The filters 45 and lifhave char-v acteristic's as illustrated inf Figures i4; and` 5 re-l spectively. The tubes, 42;:43fand55 may bez: considered; coupling tubes although they'alsofamplify the Wave-energy. The'wave energy` as modi fied by the lters 45 anddrappears on theanodes of. tubes. 19u-and; and is impressedbyi Way of impedance. couplings 5i and 52 respectively, to theinputz, electrodes of diode detectors 53- and 54- respectively. Detectors of any type may be used heres. Preferably the: detectors are: linear.; in/` character. The 'detected energies are vfed byway ofilters 55and56 tothecontrol grids of ampli-y fierrtubesf'i 'and.58.`. The filters ,55"and :5felimi nate the .intermediate frequencyy energy from the audio circuit `so thatssubstantially pure audio fre. quencyenergy appears infthe primary windings ofi itransformer i l5;-

In the graphs'of Figures 4 and `illustrating the-filter.characteristics fe is againthe carrier frequency, f1 is the lower edge of the-channel,.and ,f2 iszthe upper edge ofthe channel. As Willbe. seen; byreXamining said' graphs when the carrier increasesin-frequency` in accordance with modulati-ng potentials theloutput of filter y45- remains substantially'constant Whereas the output of lter lli-.-v increases substantially linearly as. the frequency:increases-whiletheoutput of lter 451mcreasesv substantially linearly as the frequency decreases from carrier frequency Whereas the outputof iilterfll remains substantially constant as the frequency deviates from carrier frequency.

The frequency deviation or channel Width, of the receiver of Figure 3 is not limited to twicethe signalling channel. If desired, the radio frequency channel, the intermediatey frequency channel, and the frequency deviation may be made manytimes the signal band of frequenciesl so vthat an improvement in signal-to-noise -ratio maybe effected. This is. accomplished because the noisecombines With thev carrier to. form. a resultant which is modulated inzphase and amplitude.Y The amplitude modulation isllimited .off or balanced out andl onlyl theequivalentfrequency modulationof theV phase modulation is received. Due to :the .method iny which the carrier andthe noise combine, the .noise deviates the phase of. the resultant by an. amount equal, to N-/C radians (assuming that the strengthV ofthe carrier. C isgreatscompared to the strength of thefnoise N) A `phase-deviation of N/Cr radians is equal to a frequency deviation of FmN/C cycles, Where Fmisthe beat frequency between the noiseV voltage-and the carrier. This beat frequency*l cannot 'be higher thanl the maximum signalling. frequency ldue :tothe fact that the audiochannel cutszofh the noise frequencies-;above the signalling frequencies since it is not arranged to amplify frequencies higher than the range of signalling frequencies. The applied frequency deviation due to the signal is Fd cycles-and may be several times the maximum modulation frequency, Fm, say K times greater. 'Iheratio of the frequency deviation of the signal to the frequency deviation of the noise, or in other words the signal-to-noise ratio would then be Fd-aF'mN/C, or, where Fd=KFm, would be KC/N. Thus, the normal signal-noise ratio, C/N, that would be obtained in amplitude modulation reception is multiplied by the factor K when frequency modulation is used. 'I'here is also a further improvement giving a factor of the square-roet of three or two, depending on the type of noise, which is due to the fact that the equivalent frequency deviation of the phase deviation of the noise is proportional to the audio frequency of the noise, Fm. This makes the total improvement due to frequency modulation equal to 1.73K or 2K depending upon whether the noise is fluctuation noise (tube hiss, thermal agitation, etc.) or impulse noise (ignition and most man-made noise) respectively.

While I have described my invention as being applicable to heterodyne receivers,obviouslyitmay be used equally well with receivers of th'e tuned radio frequency type. In this case the receiver will comprise a radio frequency amplier followed by a limiter which is in turn connected with coupling tubes in turn connected with the lter circuits illustrated in this invention. More preferably the transformer 6 of the various modifications illustrated may be tuned to the radio frequency wave transmitted and may be preceded by any desired number of amplifiers and an amplitude limiter.

Although in describing this receiver particular reference has been made to a frequency modulated wave and frequency modulated wave demodulating methods and means, it is obvious that the same may be adapted to the reception of phase modulation and I contemplate the use of these receivers for phase modulated wave reception. In this case, I will add or use an intermediate frequency or audio frequency correction circuitV asdescribed in Crosby United States application #124,967, led February l0, 1937, and in Crosby United States application #618,154 filed June 20, ,1932 respectively.

I claim:

1. The method of demodulating wave energy modulated in phase or frequency which includes the steps of, separating said wave energy into two portions, utilizing one of said portions to produce potentials which vary in amplitude substantially linearly as the phase or frequency of s'aid wave varies between the mean phase or frequency of said wave energy and one remote side frequency only, utilizing the other of said portions to produce potentials which vary in amplitude substantially linearly as the phase or frequency of said carrier wave varies between the mean phase or frequency of said wave energy and the other remote side frequency only, and combining said produced potentials to reproduce the signal.

2. The method of demodulating wave energy modulated in phase or frequency which includes the steps of, separating said wave energy into two portions, utilizing one of said portions to produce potentials which vary in amplitude substantially linearly as the phase or frequency of said wave energy varies from its mean phase or frequency in one direction and potentials which are of substantially constant amplitude as the phase or frequency of said wave varies Afrom its mean phase or frequencyin the other direction, utilizing the other of said portions to produce potentials which are. substantially constant as the phase or frequency of said wave varies from its mean phase or frequency in said one direction and potentials which vary substantially linearly as the phase or frequency of said wave energy Varies from its mean phase and frequency in said other direction, and combining said produced potentials to reproduce the signal.

3. The method ofidemodulating wave energy resulting from beating wave energy modulated in phase or frequency with local'oscillations to produce a new carrier and side frequencies which are characteristic of said modulated wave energy which includes the steps of, separating said new carrier and side frequencies into two portions each of which includes carrier and side frequencies, utilizing one of said portions to produce potentials which vary in amplitude substantially linearly as the phase or frequency of said carrier varies between its mean phase or frequency and one remote side frequency only, utilizing the other of said portions to produce potentials which vary substantially linearly as the phase or frequency of said carrier varies from betweenmean phase and frequency and the other remote side frequency only, and combining said produced potentials to reproduce the signal.

4. The method of demodulating Wave energy resulting from beating oscillations with wave energy modulated in phase or frequency to produce a new carrier and side frequencies characteristic of said modulated Wave energy which includesthe steps of, separating said wave energy resulting from said beating action into two portions each of which includes new carrier and both sidebands, utilizing one of said portions to produce potentials which vary in amplitude substantially linearly as the phase or frequency of said new carrier varies from its mean phase or frequency in one direction and potentials which are v of substantially constant amplitude as the phase or frequency of said newV carrier varies in the other direction, utilizing the other of said portions to produce potentials which are substantially constant as the phase or frequency of said wave varies from its mean value in said one direction and potentials which vary substantially linearly as the phase or frequency of said new carrier varies from its mean value in said other direction, and combining said produced potentials to reproduce the signal.

5. The method of dernodulation wave energy comprising carrier and sidebands resulting from modulating a carrier in phase or frequency which includes the steps of, separating said wave energy into two portions each of which includes carrier and both sidebands, utilizing to Yproduce potentials which vary in amplitude substantially linearly as the phase or frequency of said wave increases from the carrier frequency through one sideband frequency range and potentials which are of substantially constant amplitude as the phase or frequencyof said wave decreases through the other sideband range, utilizing said other portion to produce potentials which are substantially constant as the phase or frequency of said wave increases from the carrier frequency through said one sideband frequency range and potentials which vary substantially linearly as said carrier decreases in phase or frequency through said other sideband range and combining said produced potentials to reproduce the signal.

6. The method of demodulation wave energy comprising carrier and sidebands resulting from modulating a carrier in phase or frequency which includes the steps of, separating said wave energy into two portions each of which includes carrier and both sidebands, utilizing one of said portions to produce potentials which increase in amplitude substantially linearly as the phase or frequency of said wave increases from the carrier frequency through one sideband frequency range and potentials which are of substantially constant amplitude as the phase or frequency of said wave decreases through the other sideband range, utilizing said other portion to produce potentials which are of substantially zero amplitude as the phase or frequency of said wave increases from the carrier frequency through said one sideband frequency range and potentials which increase in amplitude substantially linearly as said carrier decreases in phase or frequency through said other sideband range and combining said produced potentials to reproduce the signal.

7. The method of demodulation wave energy comprising carrier and sidebands resulting from modulating a carrier in phase or frequency which includes the steps of, separating said wave energy into two portions each of which includes carrier and both sidebands, utilizing one of said portions to produce potentials which decrease in amplitude substantially linearly as the phase or frequency of said wave increases from the carrier frequency through one sideband frequency range and potentials which are of substantially constant amplitude as the phase or frequency of said wave dccreases through the other sideband range, utilizing said other portion to produce potentials which are substantially constant as the phase or frequency of said wave increases from the carrier frequency through said one sideband frequency range and potentials which decrease in amplitude substantially linearly as said carrier varies in phase or frequency through said other sideband range and combining said produced potentials to reproduce the signal.

8. The method of demodulating wave energy modulated in phase or frequency and eliminating from the demodulated output components of undesired wave energy of different frequency which includes the steps of, beating said wave energies with local oscillations of a frequency differing from the mean frequency of said modulated wave energy by a selected beat frequency, separating the energy resulting from said beating action into two portions, utilizing one of said portions to produce potential which decreases in amplitude substantially linearly as the phase or frequency of said modulated wave varies from the mean phase or frequency in one direction, utilizing the other of said portions to produce potentials which are substantially zero as the phase or frequency of said wave varies from the mean phase or frequency in said one direction and potentials which vary in amplitude substantially linearly as the phase or frequency of said carrier varies from the mean phase and frequency in said other direction, adjusting the frequency of said local oscillations to that side of the mean frequency of the modulated energy at which the interfering wave energy produces with the local oscillations a beat note which is said one direction from said carrier, and combining said produced potentials to reproduce the signal.

9. In a system for demodulating wave energy modulated in phase or frequency, a pair of filter circuits each having an input and an output, one of said filter circuits having a characteristic such that wave energy passed thereby has a substantially constant amplitude in the output thereof for all frequencies of one sideband and an output the amplitude of which varies substantially linearly through variations in frequency through the other sideband, a second filter circuit having characteristics which are opposite to the characteristic of said first named filter, means for impressing frequency or phase modulated wave energy on the inputs of said filters and demodulating means connected with the outputs of said filters.

10. In a system for demodulating wave energy modulated in phase or frequency at signal frequency, a pair of electron discharge coupling tubes each having an input electrode and an output electrode, means for impressing phase or frequency modulated wave energy on the input electrodes of said tubes, a pair of electron' discharge rectifier tubes each having input and output electrodes, a filter circuit having a characteristic such that the energy in the output thereof varies substantially linearly with variations in frequency in one direction from a selected mean frequency and is substantially constant for variations in frequency in the other direction from said mean frequency, a coupling between the input of said filter circuit and one of said coupling tubes, a coupling between the output of said filter and the input of one of said rectifiers, a second filter having a characteristic such that the output thereof is substantially constant for variations in frequency in one direction from said mean frequency and varies substantially linearly with variations in frequency in the other direction from said mean frequency, a coupling between the input of said last named filter and the output of the other of said coupling tubes, a coupling between the output of said last named filter and the input of the other of said rectifiers, and means for coupling the output of said rectiers in pushpull or push-push relation.

11. In a system for demodulating wave energy modulated in phase or frequency at signal frequency, a filter circuit having a characteristic such that all frequencies on one side of the carrier frequency are of like intensity and all frequencies on the other side of the carrier frequency are attenuated substantially linearly in accordance with their frequency, a second filter circuit having a characteristic such that all frequencies passed on said one side of the carrier frequency are attenuated substantially linearly in accordance with their frequencies and all frequencies on said other side of said carrier are passed with like intensities, means for impressing wave energy modulated in phase or frequency on the inputs of the said filters, means for rectifying the energy at the outputs of said filters and means for combining the output energies.

MURRAY G. CROSBY. 

